This invention relates to an automatic velocity calibrator for a servo system controlling head positioning for magnetic disk drives and more particularly, to a technique which allows the calibration to be performed during the time that the disk drive is performing a rezero operation.
Large quantities of digital data are stored on rotating magnetic disks. A typical disk drive in which the present invention may be used, includes one or more magnetic disks which are stacked one atop another on a common shaft and rotated at a constant speed. Magnetic read/write heads, at least one for each side of each disk, are moved in and out radially with respect to the disk in response to the machine commands. The magnetic heads are mounted on a common carriage assembly, which is driven by an actuator. A position signal specifying the relative position of the heads to the tracks is generated by a head which reads servo encoded information on a disk. In order to accurately position the heads with respect to the tracks, an accurate servo system is required. One commonly used type of servo system responds to commands which specify the velocity of the actuator and the driven carriage. The actual velocity of this carriage is sensed and compared to the specified velocity in the servo system. U.S. Pat. No. 4,321,517--Touchton and Hill shows an example of a servo system for a magnetic disk drive in which requested velocity is compared to carriage velocity in a servo system.
In such servo systems, it is necessary to calibrate the system to assure that the requested velocity is being produced. The response of a servo system to a given velocity command is affected by component aging, temperature and other environmental factors as well as being different from machine to machine. Therefore, calibration is required.
The prior art calibration techniques have not been completely satisfactory. Typically, calibration is performed only at startup of the machine. For example, the ISS/Univac model 7350 disk drive requires repeated seeks of the servo system during calibration. It is feasible to perform these only during an extended interval, such as at startup.
Most magnetic disk storage systems have an operation which is referred to as a "rezero". At startup or when an access error occurs, the data heads must be placed at a known position on the disk. This operation is commonly referred to as a "rezero" because the heads are placed over cylinder 0 on the disks. A servo error can occur because the servo is out of calibration. It would be desirable to calibrate the servo system during the rezero operation, which is normally done as part of the error recovery process, to prevent non-recoverable errors due to the system being out of calibration.
It is an object of the present invention to provide a calibrator and a calibration method which can be performed during a single pass of the magnetic heads over the disk to allow it to be performed during a rezero operation without extending the time required for servo error recovery.